Rollup merge of rust-lang#36307 - faebser:E0408_new_error_format, r=GuillaumeGomez

Changed error message E0408 to new format

Followed your text and was able to change the ouput to the new format.
I did not encounter any broken test therefore this is a really small commit.

Thanks for letting me hack on the compiler :)

r? @jonathandturner

Author: Jonathan Turner

src/librustc_resolve/lib.rs

@@ -276,13 +276,15 @@ fn resolve_struct_error<'b, 'a: 'b, 'c>(resolver: &'b Resolver<'a>,
             err
         }
         ResolutionError::VariableNotBoundInPattern(variable_name, from, to) => {
-            struct_span_err!(resolver.session,
+            let mut err = struct_span_err!(resolver.session,
                              span,
                              E0408,
                              "variable `{}` from pattern #{} is not bound in pattern #{}",
                              variable_name,
                              from,
-                             to)
+                             to);
+            err.span_label(span, &format!("pattern doesn't bind `{}`", variable_name));
+            err
         }
         ResolutionError::VariableBoundWithDifferentMode(variable_name,
                                                         pattern_number,

src/test/compile-fail/E0408.rs

@@ -12,7 +12,7 @@ fn main() {
     let x = Some(0);
 
     match x {
-        Some(y) | None => {} //~ ERROR E0408
-        _ => ()
+        Some(y) | None => {} //~  ERROR variable `y` from pattern #1 is not bound in pattern #2
+        _ => ()              //~| NOTE pattern doesn't bind `y`
     }
 }

src/test/compile-fail/issue-2848.rs

@@ -19,7 +19,7 @@ mod bar {
 fn main() {
     use bar::foo::{alpha, charlie};
     match alpha {
-      alpha | beta => {} //~ ERROR variable `beta` from pattern #2 is not bound in pattern #1
-      charlie => {}
+      alpha | beta => {} //~  ERROR variable `beta` from pattern #2 is not bound in pattern #1
+      charlie => {}      //~| NOTE pattern doesn't bind `beta`
     }
 }

src/test/compile-fail/resolve-inconsistent-names.rs

@@ -11,7 +11,9 @@
 fn main() {
     let y = 1;
     match y {
-       a | b => {} //~ ERROR variable `a` from pattern #1 is not bound in pattern #2
-       //~^ ERROR variable `b` from pattern #2 is not bound in pattern #1
+       a | b => {} //~  ERROR variable `a` from pattern #1 is not bound in pattern #2
+                   //~^ ERROR variable `b` from pattern #2 is not bound in pattern #1
+                   //~| NOTE pattern doesn't bind `a`
+                   //~| NOTE pattern doesn't bind `b`
     }
 }